This invention relates to over-pressure relief apparatus and more particularly to a dual rupture disk assembly which reliably bursts when subjected to a predetermined pressure level.
More generally, in large, continuous-process fluid pressure systems, such as are used in uranium enrichment plants, oil refineries or other process industries, the system must be protected from operating above prescribed pressure limits. Typically, pressure relief valves are provided at many locations around the pressure system. These relief valves may be similar to those shown in my U.S. Pat. No. 3,872,875 in which a valve member is resiliently held closed against a valve seat by a spring. As the system pressure increases, the spring must withstand more of the system pressure. At the set pressure of the relief valve, the forces applied to the valve member by the system pressure and by the spring are balanced so that any increase in system pressure will cause the valve member to open and to vent the system. Even at normal system operating pressures appreciably below the set pressure of the relief valve, these relief valves often tend to leak. As shown in my above-mentioned U.S. patent, reverse acting rupture disks are used to isolate such spring-operated relief valves from the system pressure and to thus provide a leak-tight system. Typically, these relief valve isolators utilize a reverse acting rupture disk having its convex face subjected to the system pressure. Upon the rupture disk being subjected to a predetermined pressure level, it will abruptly snap over center. Knife blades are provided on the back side of the rupture disk to cut it as it snaps over center thereby to subject the relief valve to the system pressure.
In other over-pressure relief systems, two prebulged rupture disks have been used in tandem. Generally, these tandem prebulged disks are mounted between a pair of piping flanges and require a pair of mounting rings each engageable with a respective piping flange and a respective prebulged rupture disk and a spacer ring between the rupture disks. One of the prebulged rupture disks, constituting a primary disk, has its concave face subjected to the system pressure so that upon the system pressure exceeding a predetermined level, it will fail thus instantaneously subjecting the other prebulged disk to the system pressure which in turn causes it to fail. The second prebulged rupture disk also has its concave face toward the system pressure and it serves as a backup to the primary disk so as to entrap any fluid which may have leaked past the primary disk. Typically, these prebulged rupture disks must be accurately mounted in precision machined mounting rings so as to insure that the prebulged rupture disks will fail at their predetermined pressure level. During installation of the prebulged disks, precise alignment of the prebulged disks in their mounting rings and accurate tightening of the bolts holding the mounting rings together are required to insure the disks will reliably rupture at their predetermined burst pressures. Also, by requiring two mounting rings and a spacer ring for mounting the prebulged rupture disks, at least two interfaces (i.e., a first interface between the outlet piping flange and the first mounting ring and a second interface between the first mounting ring and the primary prebulged rupture disk) must be made leak tight.
Generally, with prebulged rupture disks the maximum operating pressure of the system is limited to 70% of the minimum burst pressure of the prebulged disks. This means that the pressure system must be significantly over-designed to accommodate the increase in pressure from its maximum operating pressure until the minimum burst pressure of the prebulged rupture disk is attained.